Anode for oxygen evolution in electrolytes containing fluorides or fluoride-complex anions

ABSTRACT

The invention discloses a new electrode suitable for use as an anode for oxygen evolution from electrolytes containing fluorides or fluoride-complex anions even in high concentrations. 
     The anode of the invention comprises a titanium substrate provided with a protective interlayer resistant to the aggressive action of fluorides, and an electrocatalytic coating for oxygen evolution. 
     The protective interlayer is made of tungsten, oxides or oxyfluorides, optionally containing metals of the platinum group in minor quantities, metallo-ceramic compounds and intermetallic compounds either per se or as mixed oxides.

DESCRIPTION OF THE INVENTION

In the electrometallurgical field, the use of activated titanium anodes, made of a titanium substrate provided with a suitable electrocatalytic coating, is presently limited to a few specific applications such as chromium plating from conventional baths and gold plating.

The active coating may be alternatively based on:

a) platinum (mainly obtained by galvanic deposition)

b) noble metal oxides (mainly obtained by thermal treatment).

Both coatings are satisfactorily performing in sulphuric acid or similar solutions, provided that no fluorides or fluoride-containing anions are present, as it happens with the chromium deposition from conventional electrolytes, where the anodic lifetime reaches three years or more with electrode potentials 0.5 to 1.5 V lower than those typical of lead anodes. Conversely, they find no industrial application in electrolytes containing fluorides. In fact, even small contents of fluorides, in the range of one part per million (hereinafter ppm), irreversibly de-stabilize the anode (maximum lifetime of a few weeks only). It must be noted that the average concentration in industrial electrolytes may vary from some tens of parts per million (ppm) to some grams per liter (g/l). The destabilization of the anode is substantially due to the corrosion of the titanium substrate caused by the fluorides or fluoride-complex anions which make the titanium oxides soluble.

The complexing action of fluorides and fluoride-containing anions, which takes place according to an increasing order as follows: AlF₆ ³⁻, FeF₆ ³⁻, <SiF₆ ²⁻ <BF₄ ⁻ <HF₂ ⁻ <F⁻, is accelerated by acidity and temperature.

The presence of fluorides or fluoride-containing anions is normal in electrolytes of many industrial processes, where they are either added to, with the aim of obtaining particular characteristics of the deposited metal, as well as improving deposition speed and penetrating power, or released by the leached minerals.

It has been found that the use of titanium as a substrate for anodes suitable for electrolytes containing fluorides is possible if titanium is subjected, prior to the application of the electrocatalytic coating, to a pre-treatment comprising applying on its surface an interlayer made of elements or compounds potentially stable under the required operating conditions.

The selection criteria for the interlayer characteristics, (components and percentages) and the coating application or formation methods are reported in Tables 1 and 2.

                                      TABLE 1                                      __________________________________________________________________________     Interlayer selection criteria                                                  __________________________________________________________________________     1. Fluoride-resistant metals, alloys or oxides thereof, e.g. noble metals      (Pt, Pd                                                                         etc.), mixtures or alloys thereof (Pt--Ir, Pt--Pd ,etc.) and tungsten         2. Oxides or metals convertible to insoluble fluorides or oxyfluorides,        e.g.                                                                            CeO.sub.2, Cr.sub.2 O.sub.3.                                                  3. Oxides resistant to fluorides or convertible to stable fluorides or         oxyfluorides,                                                                   containing definite quantities of noble metals, optionally as mixtures,       to                                                                              enhance electroconductivity.                                                  4. Metallo-ceramic compounds, both electroconductive, due to the metal          component, and resistant to fluorides, due to the ceramic part, such as        chromium - chromium oxide.                                                    5. Electroconductive and fluoride-resistant intermetallic compounds, such      as                                                                              titanium nitride (TiN), titanium nitride (TiN) + titanium carbide             (TiC),                                                                          tungsten silicide, titanium silicide.                                         __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     Method of production of the interlayer                                         Type    Composition    Deposition procedure                                    __________________________________________________________________________     Noble   Pt 100%        Thermal decomposition of                                metals, Pd 100%        precursor salts based on chlorine                       optionally as                                                                          Pt--Ir (10-30-50%)                                                                            complexes soluble in diluted                            mixed   Pt--Pd         aqueous hydrochloric acid                               oxides or as                                                                           Pt--Ir 30%     Thermal decomposition of                                alloys  Pt--Pd 70%     isomorphous precursor salts such                                               as (NH.sub.4).sub.2 Pt(Ir)Cl.sub.6,                                            (NH.sub.3).sub.2 Pt(Pd)(NO.sub.2).sub.2                 Oxides  Cr.sub.2 O.sub.3                                                                              Plasma jet deposition of                                                       preformed oxide powder                                  Composite                                                                              TiO.sub.2 --Ta.sub.2 O.sub.5 --NbO.sub.2 (Molar                                               Thermal decomposition of                                oxides  ratio: Ti 75, Ta 20, Nb 5);                                                                   precursor salts based on                                        TiO.sub.2 --Ta.sub.2 O.sub.5 --CeO.sub.2 (Molar                                               chlorometallates soluble in a                                   ratio: Ti 75,Ta 20 ,Ce 5);                                                                    concentrated hydrochloric solution                              TiO.sub.2 --Ta.sub.2 O.sub.5 --Cr.sub.2 O.sub.3                                               (HCl ≧ 10%)                                              ratio: Ti 75, Ta 20, Cr 5)                                             Composite                                                                              TiO.sub.2 --Ta.sub.2 O.sub.5 --IrO.sub.2 (Molar                                               Thermal decomposition of                                oxides with                                                                            ratio: Ti 75, Ta 20, Ir 5;                                                                    precursor salts based on                                low content                                                                            Ti 70, Ta 20, Ir 10); TiO.sub.2 --                                                            chlorocomplexes soluble in                              of noble                                                                               Ta.sub.2 O.sub.5 --Nb.sub.2 O.sub.5 --IrO.sub.2                                               aqueous hydrochloric acid (≧10%)                 metal   ratio: Ti 70, Ta 20, Nb5, Ir 5)                                        Metallo-                                                                               Cr (2 microns) - Cr.sub.2 O.sub.3                                                             Galvanic chromium deposition                            ceramic Cr (20 microns) - Cr.sub.2 O.sub.3                                                            from a conventional sulphate bath                       compounds              and thermal post-oxidation in air                                              (450° C. - 1 hour).                              Simple  TiN            Plasma jet deposition from a pre-                       intermetallic          formed powder                                           compounds                                                                              TiN            Ionic nitridization                                             TiN            Nitridization in ammonia (600° C.,                                      3 hours, 10 atm)                                        Composite                                                                              TiN + TiC      Carbo-nitridization from molten                         intermetallic          salts                                                   compounds                                                                      __________________________________________________________________________

The invention will be better illustrated by means of some examples wherein samples having the dimensions of 40 mm×40 mm×2 mm, made of titanium grade 2, have been prepared as follows:

a) Surface pretreatment by sandblasting with aluminum oxide powder+pickling in 20% HCl, 30 minutes;

b) application of the protective interlayer;

application of the electrocatalytic coating for oxygen evolution. The samples have been characterized by means of measurement of the electrochemical potential when used as anodes in electrolytes simulating the same operating conditions as in industrial processes and comparison of the results with reference samples prepared according to the prior art teachings.

EXAMPLE 1

No. 64 reference titanium samples, prepared according to the prior art teachings, dimensions 40 mm×40 mm×2 mm each, were subjected to a surface pre-treatment following the procedures mentioned above in item a).

Then, 32 samples, identified by A, were directly activated with an electrocatalytic coating made of Ta--Ir (Ir 64% molar and about the same by weight) and 32 samples, identified by B, were provided with an interlayer based on Ti--Ta (Ta 20% molar) and then with an electrocatalytic coating made of Ta--Ir (Ir 64% molar).

The compositions of the paints are reported in the following table:

    __________________________________________________________________________     Paint characteristics                                                                   Interlayer    Electrocatalytic coating                                __________________________________________________________________________     Component                                                                               TiCl.sub.3 TaCl.sub.5                                                                 HCl (20%)                                                                             TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                                                        HCl (20%)                                      Content - mg/cc                                                                         5.33 (Ti)                                                                             5.03 (Ta)                                                                             50 (Ta)  90 (Ir)                                        as metal                                                                       __________________________________________________________________________

The composition of the layers is described in the following table:

    __________________________________________________________________________     Characteristics                                                                               Stabilizing interlayer                                                                    Electrocatalytic coating                             __________________________________________________________________________     Components     Ta.sub.2 O.sub.5 --TiO.sub.2                                                              Ta.sub.2 O.sub.5 IrO.sub.2                           % molar as metal                                                                              20   80    36   64                                              g/m.sup.2 as metal or noble metal                                                             Σ1.0 10                                                   __________________________________________________________________________

The interlayer was applied by brushing the paint. The application was repeated until the desired load was obtained (1.0 g/m² total metal). Between one application and the subsequent one the paint is subjected to drying at 150° C., followed by thermal decomposition in oven under forced air circulation at 500° C. for 10-15 minutes and subsequent natural cooling.

On the protective interlayer the electrocatalytic coating is applied, also by brushing or equivalent technique. The application is repeated until the desired final load is obtained (10 g/m² as noble metal). Between one application and the subsequent one the paint is subjected to drying at 150° C., followed by thermal decomposition in oven under forced air circulation at 500° C. for 10-15 minutes and subsequent natural cooling.

EXAMPLE 2

16 electrode samples having the same dimensions as those of Example 1 were prepared according to the present invention, applying various interlayers based on mixed oxides belonging to the transition metals and lanthanides. The samples were pre-treated (sandblasting+pickling) as described in Example 1. The samples were prepared according to the following procedure

a) application of the interlayer based on mixed oxides belonging to groups IIIB, IVB, VB, VIB, VIIB and lanthanides, by thermal decomposition of solutions containing the precursor salts of the selected elements.

b) application of the electrocatalytic coating based on tantalum and iridium oxides by thermal decomposition of solutions containing the precursor salts of the selected elements as summarized in Table 2.1

                                      TABLE 2.1                                    __________________________________________________________________________     Interlayer                Electrocatalytic coating                             Sample                                                                             Components            Components                                           No. Type and %(*)                                                                          g/m.sup.2 (**)                                                                       Method  Type, %(*)                                                                           Method                                         __________________________________________________________________________     2.1 Ti--Ta--Y                                                                              1.0   Thermal Ta--Ir (64)                                                                          thermal de-                                    a, b,                                                                              (75)-(20)-(5) decomposition composition                                    c, d              from salts    from same                                                        based on      precursor                                                        chlorides or  salts as in                                                      chlorocomplex Example 1                                                        anions                                                       2.2 Ti--Ta--Cr                                                                             1.0   Thermal Ta--Ir (64)                                          a, b,                                                                              (75)-(20)-(5) decomposition                                                c, d              from salts                                                                     based on                                                                       chlorides or                                                                   chlorocomplex                                                                  anions                                                       2.3 Ti--Ta--Ce                                                                             1.0   Thermal Ta--Ir (64)                                          a, b,                                                                              (75)-(20)-(5) decomposition                                                c, d              from salts                                                                     based on                                                                       chlorides or                                                                   chlorocomplex                                                                  anions                                                       2.4 Ti--Ta--Nb                                                                             1.0   Thermal Ta--Ir (64)                                          a, b,                                                                              (75)-(20)-(5) decomposition                                                c, d              from salts                                                                     based on                                                                       chlorides or                                                                   chlorocomplex                                                                  anions                                                       2.5 Ti--Ta--Cr--                                                                           1.0   Thermal Ta--Ir (64)                                          a, b,                                                                              Nb            decomposition                                                c, d                                                                               (70)-(20)-(3)-                                                                               from salts                                                       (7)           based on                                                                       chlorides or                                                                   chlorocomplex                                                                  anions                                                       __________________________________________________________________________      (*) % molar referred to the elements at the metallic state                     (**) (g/m.sup.2) total quantity of the metals applied                    

The paints are described in Table 2.2.

                  TABLE 2.2                                                        ______________________________________                                         Description of the paints                                                      Interlayer          Electrocatalytic coating                                   Sample           % as                 % as                                     No.    components                                                                               metal  mg/cc components                                                                             metal                                                                               mg/cc                               ______________________________________                                         2.1    TaCl.sub.5                                                                               20     5.54  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               75     5.50  IrCl.sub.3                                                                             64   90                                         YCl.sub.3  5     0.68  HCl     //   110                                        HCl       //     110                                                    2.2    TaCl.sub.5                                                                               20     5.54  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               75     5.50  IrCl.sub.3                                                                             64   90                                         CrO.sub.3  5     0.40  HCl     //   110                                        HCl       //     110                                                    2.3    TaCl.sub.5                                                                               20     5.03  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               75     5.00  IrCl.sub.3                                                                             64   90                                         CeCl.sub.3                                                                                5     0.97  HCl     //   110                                        HCl       //     110                                                    2.4    TaCl.sub.5                                                                               20     5.03  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               75     5.00  IrCl.sub.3                                                                             64   90                                         NbCl.sub.5                                                                                5     0.65  HCl     //   110                                        HCl       //     110                                                    2.5    TaCl.sub.5                                                                               20     5.40  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               70     5.00  IrCl.sub.3                                                                             64   90                                         CrO.sub.3  3     0.24  HCl     //   110                                        NbCl.sub.5                                                                                7     0.97                                                          HCl       //     110                                                    ______________________________________                                    

The method of preparation of the interlayer is described in Table 2.3.

                                      TABLE 2.3                                    __________________________________________________________________________     Preparation of the interlayer                                                  __________________________________________________________________________      application of the paint containing the precursor salts by brushing or        equivalent                                                                      technique                                                                      drying at 150° C. and thermal decomposition of the paint at            500° C. for 10-15                                                        minutes in oven under forced air circulation and subsequent natural           cooling                                                                         repeating the application as many times as necessary to obtain the            desired load                                                                    (1.0 g/m.sup.2).                                                              __________________________________________________________________________

The method for applying the electrocatalytic coating was the same as described in Example 1.

The samples thus prepared were subjected to electrochemical characterization as anodes in four types of electrolytes simulating the industrial operating conditions as shown in Table 2.4. For each type of operating conditions a comparison was made using reference samples prepared as described in Example 1.

                                      TABLE 2.4                                    __________________________________________________________________________     Electrochemical characterization                                               Samples        Operating conditions                                                                           Simulated                                       Series                                                                              No.       Electrolyte                                                                             Parameters                                                                            industrial process                              __________________________________________________________________________     M    Present invention                                                                        H.sub.2 SO.sub.4 150 g/l                                                                500 A/m.sup.2                                                                         Secondary zinc                                       from 2.1a→2.5a                                                                    HF 50 ppm       and copper                                           reference samples: 40° C.                                                                         electrometallurgy                                    A1,B1                                                                     N    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                                500 A/m.sup.2                                                                         Primary copper                                       from 2.1b→2.5b                                                                    HF 300 ppm      electrometallurgy                                    reference samples: 40° C.                                               A2,B2                                                                     O    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                                1000 A/m.sup.2                                                                        Chromium plating                                     from 2.1c→2.5c                                                                    H.sub.2 SiF.sub.6 1000                                               reference samples:                                                                       ppm      60° C.                                               A3,B3                                                                     P    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                                5000 A/m.sup.2                                                                        High speed                                           from 2.1d→2.5d                                                                    H.sub.2 SiF.sub.6 1500                                                                         chromium plating                                     reference samples:                                                                       ppm      60° C.                                               A4,B4                                                                     __________________________________________________________________________

The characterization comprised:

detecting the electrode potential as a function of the operating time

detecting the possible noble metal loss at the end of the test

visual inspection.

The results are summarized in Table 2.5.

                  TABLE 2.5                                                        ______________________________________                                         Results of the electrochemical characterization                                         Potential V(NHE)                                                      Electrolyte                                                                           Samples initial                                                                               100 h                                                                               1000 h                                                                               3000 h                                                                               Morphology                              ______________________________________                                         M      2.1a    1.62   1.68 1.80  2.01  No variation                                   2.2a    1.60   1.70 1.80  1.80  "                                              2.3a    1.56   1.65 1.70  1.75  "                                              2.4a    1.58   1.64 1.70  1.69  "                                              2.5a    1.58   1.65 1.68  1.70  "                                              A1      1.63   2.81             Corrosion                                      B1      1.67   2.61             Corrosion                               N      2.1b    1.60   1.70 1.90  2.40  Corrosion                                      2.2b    1.58   1.60 1.85  1.95  No variation                                   2.3b    1.62   1.65 1.75  1.85  "                                              2.4b    1.63   1.70 1.83  1.90  "                                              2.5b    1.61   1.65 1.70  1.75  "                                              A2      1.69   2.81             Corrosion                                      B2      1.67   2.61             Corrosion                               O      2.1c    1.78   1.84 2.03  >2.6  Corrosion                                      2.2c    1.75   1.80 1.85  1.90  No variation                                   2.3c    1.65   1.65 1.75  1.75  "                                              2.4c    1.60   1.70 1.72  1.80  "                                              2.5c    1.65   1.64 1.65  1.67  "                                              A3      1.65   3.22             Corrosion                                      B3      1.72   3.47             Corrosion                               P      2.1d    1.85   1.90 2.15  4.50  Corrosion                                      2.2d    1.80   1.85 2.00  3.50  "                                              2.3d    1.78   1.85 1.90  2.20  Initial Corrosion                              2.4d    1.75   1.77 1.84  2.00  "                                              2.5d    1.84   1.85 1.97  2.20  "                                              A4      1.87   >6.0             Corrosion                                      B4      1.92   >4.5             Corrosion                               ______________________________________                                    

The results reported in Table 2.5 point out that the presence of small quantities of metal oxides, which form insoluble compounds in the electrolyte containing fluorides or fluoride-complex anions, increases the lifetime of the electrode of the invention in any operating condition.

EXAMPLE 3

24 samples, same as those of Example 2 with the only exception that the interlayers contained minor amounts of noble metals, after sandblasting and pickling, were prepared according to the following procedure:

a) application of the interlayer based on valve metal oxides containing minor amounts of noble metals, by thermal decomposition of aqueous solutions containing the precursor salts of the selected elements.

b) application of the electrocatalytic coating based on tantalum and iridium oxides applied by thermal decomposition of solutions containing the precursor salts of said elements as summarized in Table 3.1.

                                      TABLE 3.1                                    __________________________________________________________________________     Interlayer               Electrocatalytic coating                              Components               Components                                                          g/m.sup.2  Type and                                              Samples No.                                                                          Type and %(*)                                                                          (**)                                                                               Method %(*)   Method                                         __________________________________________________________________________     3.1 a, b, c, d                                                                       Ta--Ti--Ir                                                                             2.0 thermal                                                                               Ta--Ir (64%)                                                                          Thermal                                              (20)-(77.5)-(2.5)                                                                          decomposition decomposition                                                    of precursors in                                                                             from precursor                                                   hydrochloric  salt paints,                                                     solution      same as in                                                                     Example 1                                      32 a, b, c, d                                                                        Ta--Ti--Ir                                                                             2.0 thermal                                                            (20)-(75)-(5)                                                                              decomposition                                                                  or precursors in                                                               hydrochloric                                                                   solution                                                     3.3 a, b, c, d                                                                       Ta--Ti--Ir                                                                             2.0 thermal                                                            (20)-(70)-(10)                                                                             decomposition                                                                  or precursors in                                                               hydrochloric                                                                   solution                                                     3.4 a, b, c, d                                                                       Ta--Ti--Pd                                                                             2.0 thermal                                                            (15)-(80)-(5)                                                                              decomposition                                                                  or precursors in                                                               hydrochloric                                                                   solution                                                     3.5 a, b, c, d                                                                       Ta--Ti--Ir--Pd                                                                         2.0 thermal                                                            (20)-(75)-(2.5)                                                                            decomposition                                                      (2.5)       or precursors in                                                               hydrochloric                                                                   solution                                                     3.6 a, b, c, d                                                                       Ta--Ti--Nb--Ir                                                                         2.0 thermal                                                            (20)-(70)-(5)-(5)                                                                          decomposition                                                                  or precursors in                                                               hydrochloric                                                                   solution                                                     __________________________________________________________________________      (*) % molar referred to the elements at the metallic state                     (**) (g/m.sup.2) total quantity of the metals applied                    

The paints are described in Table 3.2.

                  TABLE 3.2                                                        ______________________________________                                         12/21 Paint characteristics                                                    Interlayer          Electrocatalytic coating                                   Sample           % as                 % as                                     No.    Components                                                                               metal  mg/cc Components                                                                             metal                                                                               mg/cc                               ______________________________________                                         3.1    TaCl.sub.5                                                                               20     5.30  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               77.5   5.50  IrCl.sub.3                                                                             64   90                                         IrCl.sub.3                                                                               2.5    0.70  HCl     //   110                                        HCl       //     110                                                    3.2    TaCl.sub.5                                                                               20     5.54  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               75     5.50  IrCl.sub.3                                                                             64   90                                         IrCl.sub.3                                                                               5.0    1.47  HCl     //   110                                        HCl       //     110                                                    3.3    TaCl.sub.5                                                                               20     5.94  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               70     5.50  IrCl.sub.3                                                                             64   90                                         IrCl.sub.3                                                                               10.0   3.15  HCl     //   110                                        HCl       //     110                                                    3.4    TaCl.sub.5                                                                               20     3.54  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               70     5.00  IrCl.sub.3                                                                             64   90                                         PdCl.sub.2                                                                               10     0.69  HCl     //   110                                        HCl       //     110                                                    3.5    TaCl.sub.5                                                                               20     5.54  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               75     5.50  IrCl.sub.3                                                                             64   90                                         IrCl.sub.3                                                                               2.5    0.67  HCl     //   110                                        PdCl.sub.2                                                                               2.5    0.37                                                          HCl       //     110                                                    3.6    TaCl.sub.5                                                                               20     5.40  TaCl.sub.5                                                                             36   50                                  a, b, c, d                                                                            TiCl.sub.4                                                                               70     5.00  IrCl.sub.3                                                                             64   90                                         NbCl.sub.5                                                                               5      0.69  HCl     //   110                                        IrCl.sub.3                                                                               5      1.43                                                          HCl       //     110                                                    ______________________________________                                    

The method of preparation of the interlayer is described in Table 3.3.

                                      TABLE 3.3                                    __________________________________________________________________________     Preparation of the interlayer                                                  __________________________________________________________________________      application of the paint containing the precursor salts by brushing or        equivalent                                                                      technique                                                                      drying at 150° C. and thermal decomposition of the paint at            500° C. for 10-15                                                        minutes in oven under forced air circulation and subsequent natural           cooling                                                                         repeating the application as many times as necessary to obtain the            desired load                                                                    (2 g/m.sup.2).                                                                __________________________________________________________________________

The method for applying the electrocatalytic coating was the same as described in Example 1.

The samples thus prepared were subjected to electrochemical characterization as anodes in four types of electrolytes simulating the industrial operating conditions as shown in Table 3.4. For each type of operating conditions a comparison was made using reference samples prepared as described in Example 1. In particular, in addition to the reference electrodes as described in Example 1, also the best electrode sample of Example 2 (namely sample 2.4) was compared with the present samples.

                                      TABLE 3.4                                    __________________________________________________________________________     Electrochemical characterization                                               Sample         Operating conditions                                                                          Simulated                                        Series                                                                              No.       Electrolyte                                                                            Parameters                                                                            industrial process                               __________________________________________________________________________     M    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                                500 A/m.sup.2                                                                        Secondary zinc and                                    from 3.1a → 3.6a                                                                  HF 50 ppm                                                                              40° C.                                                                         copper                                                reference samples:       electrometallurgy                                     A5, B5, 2.4                                                               N    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                                500 A/m.sup.2                                                                        Primary copper                                        from 3.1b → 3.6b                                                                  HF 300 ppm                                                                             40° C.                                                                         electrometallurgy                                     reference samples:                                                             A6, B6, 2.4                                                               O    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                               1000 A/m.sup.2                                                                        Conventional                                          from 3.1c → 3.6c                                                                  H.sub.2 SiF.sub.6 1000                                                                 60° C.                                                                         chromium plating                                      reference samples:                                                                       ppm                                                                  A7, B7, 2.4                                                               P    Present invention:                                                                       H.sub.2 SO.sub.4 150 g/l                                                               5000 A/m.sup.2                                                                        High speed                                            from 3.1d → 3.6d                                                                  H.sub.2 SiF.sub.6 1500                                                                 60° C.                                                                         chromium plating                                      reference samples:                                                                       ppm                                                                  A8, B8, 2.4                                                               __________________________________________________________________________

The characterization comprised detecting the electrode potential as a function of the operating time, detecting the possible noble metal loss at the end of the test and visual inspection.

The results are summarized in Table 3.5.

                  TABLE 3.5                                                        ______________________________________                                         Results of the electrochemical characterization                                         Potential V(NHE)                                                      Electrolyte                                                                           Samples initial                                                                               100 h                                                                               1000 h                                                                               3000 h                                                                               Morphology                              ______________________________________                                         M      3.1a    1.60   1.78 1.83  2.12  No variation                                   3.2a    1.69   1.70 1.72  1.73  "                                              3.3a    1.60   1.71 1.70  1.70  "                                              3.4a    1.58   1.65 1.66  1.67  "                                              3.5a    1.60   1.61 1.64  1.64  "                                              3.6a    1.64   1.63 1.65  1.70  "                                              2.4     1.58   1.64 1.70  1.69  "                                              A5      1.63   3.15             Corrosion                                      B5      1.66   2.19             Corrosion                               N      3.1b    1.64   1.79 1.98  2.35  Corrosion                                      3.2b    1.63   1.74 1.78  1.79  No variation                                   3.3b    1.64   1.70 1.75  1.74  "                                              3.4b    1.62   1.68 1.68  1.72  "                                              3.5b    1.62   1.64 1.65  1.69  "                                              3.6b    1.66   1.71 1.75  1.80  "                                              2.4     1.63   1.70 1.83  1.90  "                                              A6      1.63   2.75             Corrosion                                      B6      1.67   2.31             Corrosion                               O      3.1c    1.77   1.83 1.97  >2.5  Corrosion                                      3.2c    1.75   1.75 1.83  1.91  No variation                                   3.3c    1.76   1.75 1.78  1.82  "                                              3.4c    1.74   1.75 1.75  1.80  "                                              3.5c    1.75   1.76 1.75  1.76  "                                              3.6c    1.81   1.87 1.89  1.91  "                                              2.4     1.60   1.70 1.72  1.80  "                                              A7      1.68   3.19             Corrosion                                      B7      1.79   2.66             Corrosion                               P      3.1d    1.86   1.89 2.12  4.6   Corrosion                                      3.2d    1.81   1.85 1.97  2.9   "                                              3.3d    1.80   1.82 1.94  2.15  Initial corrosion                              3.4d    1.79   1.79 1.87  2.10  "                                              3.5d    1.78   1.79 1.83  2.06  "                                              3.6d    1.89   1.95 1.99  2.18  "                                              2.4     1.75   1.77 1.84  2.00                                                 A8      1.90   >6.0             Corrosion                                      B8      1.92   >5.0             Corrosion                               ______________________________________                                    

The analysis of the results reported in Table 3.5 leads to the conclusion that the presence of noble metals in the interlayer, mainly consisting of transition metal oxides, increases the lifetime of the electrodes of the invention in any type of solutions.

EXAMPLE 4

16 electrode samples having the same dimensions as those of Example 1 were prepared according to the present invention, comprising various metallo-ceramic (cermet) interlayers based on chromium and chromium oxide. The samples were prepared according to the following procedure:

galvanic chromium deposition

controlled oxidation with formation of a protective metallo-ceramic interlayer

subsequent application of the electrocatalytic coating based on tantalum and iridium.

The method of preparation and the characteristics of the samples are described in Table 4.1.

                  TABLE 4.1                                                        ______________________________________                                         Interlayer                                                                                  Average                                                           Sample           thickness                                                                               Air oxidation                                                                           Electrocatalytic                            No.    Method    (micron) (hours)                                                                              (° C.)                                                                       coating                                   ______________________________________                                         4.1    H.sub.2 SO.sub.4 3.5                                                                     1        //    //   Ta--Ir (64%) by                           a, b, c, d                                                                            g/l                           thermal                                          CrO.sub.3 300 g/l             decomposition                                    65° C.                 from precursor                                   1000 A/m.sup.2                salt paints, as in                                                             Example 1                                 4.2    H.sub.2 SO.sub.4 3.5                                                                     1        1/2   400  Ta--Ir (64%) by                           a, b, c, d                                                                            g/l                           thermal                                          CrO.sub.3 300 g/l             decomposition                                    65° C.                 from precursor                                   1000 A/m.sup.2                salt paints, as in                                                             Example 1                                 4.3    H.sub.2 SO.sub.4 3.5                                                                     1        1/2   450  Ta--Ir (64%) by                           a, b, c, d                                                                            g/l                           thermal                                          CrO.sub.3 300 g/l             decomposition                                    65° C.                 from precursor                                   1000 A/m.sup.2                salt paints, as in                                                             Example 1                                 4.4    H.sub.2 SO.sub.4 3.5                                                                     3        1/2   450  Ta--Ir (64%) by                           a, b, c, d                                                                            g/l                           thermal                                          CrO.sub.3 300 g/l             decomposition                                    65° C.                 from precursor                                   1000 A/m.sup.2                salt paints, as in                                                             Example 1                                 ______________________________________                                    

The samples thus prepared were subjected to anodic electrochemical characterization in four types of electrolytes simulating the industrial operating conditions as shown in Table 4.2. For each type of operating conditions a comparison was made using reference samples prepared according to the prior art teachings as described in Example 1.

                  TABLE 4.2                                                        ______________________________________                                         Electrochemical characterization                                                                                  Operating                                   Series                                                                               Sample No.      Electrolyte  conditions                                  ______________________________________                                         M     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   4.1a→4.4a,                                                                              HF      50 ppm 40° C.                                   reference samples:                                                             A9, B9                                                                   N     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   4.1b→4.4b,                                                                              HF      300 ppm                                                                               50° C.                                   reference samples:                                                             A10, B10                                                                 O     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               1000 A/m.sup.2                                  4.1c→4.4c,                                                                              H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples:                                                             A11. B11                                                                 P     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               5000 A/m.sup.2                                  4.1d→4.4d,                                                                              H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples                                                              A12, B12                                                                 ______________________________________                                    

The characterization comprised detecting the electrode potential as a function of the operating time, detecting the possible noble metal loss at the end of the test and visual inspection.

The results are summarized in Table 4.3.

                  TABLE 4.3                                                        ______________________________________                                         Results of the electrochemical characterization                                         Potential (V(NHE)                                                     Electrolyte                                                                           Samples initial 100 h 1000 h                                                                               3000 h                                                                               Morphology                            ______________________________________                                         M      4.1a    1.81    >3.0              Corrosion                                    4.2a    1.75    1.75  >3.0        Corrosion                                    4.3a    1.74    1.74  1.75  1.89  No variation                                 4.4a    1.78    1.76  1.76  1.79  "                                            A9      1.62    2.90              Corrosion                                    B9      1.65    2.31              Corrosion                             N      4.1b    1.83    >4.0              Corrosion                                    4.2b    1.77    1.98  >3.6        Corrosion                                    4.3b    1.75    1.77  1.78  1.89  No variation                                 4.4b    1.78    1.79  1.82  1.83  "                                            A10     1.63    2.98              Corrosion                                    B10     1.67    2.22              Corrosion                             O      4.1c    1.89    >5.0              Corrosion                                    4.2c    1.86    1.86  >2.5        Corrosion                                    4.3c    1.83    1.84  1.85  1.91  No variation                                 4.4c    1.82    1.84  1.85  1.86  "                                            A11     1.68    3.12              Corrosion                                    B11     1.75    2.55              Corrosion                             P      4.1d    1.93    >5.0              Corrosion                                    4.2d    1.90    1.92  >2.5        Corrosion                                    4.3d    1.88    1.88  1.89  1.94  No variation                                 4.4d    1.87    1.87  1.87  1.90  "                                            A12     1.84    >5.5              Corrosion                                    B12     1.89    >4.0              Corrosion                             ______________________________________                                    

The analysis of the results leads to the conclusion that the electrodes of the invention obtained by galvanic deposition and thermal oxidation are more stable than those of the prior art. In particular this stability (corrosion resistance, weight loss and potential with time) increases according to the following order, depending on the type of substrate:

    __________________________________________________________________________     Cr   < Cr + oxidation                                                                         < Cr + oxidation                                                                         < Cr + oxidation                                      1 micron                                                                              1 micron 400° C.                                                                  1 micron 450° C.                                                                  3 micron 450° C.                             __________________________________________________________________________

EXAMPLE 5

12 electrode samples comprising various interlayers based on titanium nitride and having the same dimensions as those of Example 1 were prepared following the same pretreatment procedure described in Example 1. Nitridization was subsequently carried out by in-situ formation of a protective titanium nitride interlayer and the electrocatalytic coating was then applied (Table 5.1). The in situ formation was obtained by the conventional thermal decomposition technique of reactant gases or by ionic gas deposition.

                  TABLE 5.1                                                        ______________________________________                                         Method of forming the interlayer and the electrocatalytic coating              Interlayer                                                                     Sample Compo-  Thickness            Electrocatalytic                           No.    sition  (micron) Method      coating                                    ______________________________________                                         5.1a,b,c,d                                                                            TiN     3-3.1    Plasma jet deposition                                                                      Ta--Ir (64%),                                                      of TiN powder (0.5-                                                                        Thermal                                                            1.0 micron) decomposition                                                                  from precursor                                                                 salt paints, as                                                                in Example 1                               5.2a,b,c,d                                                                            TiN     2.9-3.0  "in situ" formation                                                                        Ta--Ir (64%),                                                      by ionic nitridization:                                                                    Thermal                                                            gas: N.sub.2                                                                               decomposition                                                      pressure: 3-10 millibar                                                                    from precursor                                                     temperature: 580° C.                                                                salt paints, as                                                                in Example 1                               5.3a,b,c,d                                                                            TiN     2.9-3.1  "in situ" formation by                                                                     Ta--Ir (64%),                                                      gas nitridization:                                                                         Thermal                                                            gas: NH.sub.3                                                                              decomposition                                                      catalyst: palladiate                                                                       from precursor                                                     carbon      salt paints, as                                                    pressure: 3-4 atm                                                                          in Example 1                                                       temperature: 580° C.                            ______________________________________                                    

The samples thus prepared were subjected to electrochemical characterizations anodes in four types of electrolytes simulating the industrial operating conditions as shown in Table 5.2. For each type of operating conditions a comparison was made using reference samples prepared according to the prior art teachings as described in Example 1.

                  TABLE 5.2                                                        ______________________________________                                         Electrochemical characterization                                                                                  Operating                                   Series                                                                               Sample No.      Electrolyte  Conditions                                  ______________________________________                                         M     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   5.1a→5.3a,                                                                              HF      50 ppm 40° C.                                   reference samples:                                                             A13, B13                                                                 N     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   5.1b→5.3b,                                                                              HF      300 ppm                                                                               50° C.                                   reference samples:                                                             A14, B14                                                                 O     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               1000 A/m.sup.2                                  5.1c→5.3c,                                                                              H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples:                                                             A15, B15                                                                 P     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               5000 A/m.sup.2                                  5.1d→5.3d                                                                               H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples:                                                             A16, B16                                                                 ______________________________________                                    

The characterization comprised:

detecting the electrode potential as a function of the operating time

detecting the possible noble metal loss at the end of the test

visual inspection.

The results are summarized in Table 5.3.

                  TABLE 5.3                                                        ______________________________________                                         Results of the characterization                                                         Potential (V(NHE)                                                     Electrolyte                                                                           Samples initial 100 h 1000 h                                                                               3000 h                                                                               morphology                            ______________________________________                                         M      5.1a    1.8     1.81  1.81  1.84  No variation                                 5.2a    1.78    1.79  1.79  1.81  "                                            5.3a    1.83    1.84  1.88  1.85  "                                            A13     1.63    3.05              Corrosion                                    B13     1.66    2.44              Corrosion                             N      5.1b    1.83    1.83  1.86  1.89  No variation                                 5.2b    1.79    1.82  1.84  1.86  "                                            5.3b    1.85    1.85  1.91  1.95  "                                            A14     1.62    2.87              Corrosion                                    B14     1.68    2.25              Corrosion                             O      5.1c    1.87    1.87  1.89  1.93  No variation                                 5.2c    1.85    1.84  1.85  1.90  "                                            5.3c    1.91    1.93  1.98  2.08  Initial                                                                        corrosion                                    A15     1.65    3.23              Corrosion                                    B15     1.73    2.57              Corrosion                             P      5.1d    1.90    1.91  1.92  1.95  No variation                                 5.2d    1.88    1.88  1.89  1.90  Initial                                                                        corrosion                                    5.3d    1.93    1.98  2.05  2.12  Initial                                                                        corrosion                                    A16     1.82    >5.5              Corrosion                                    B16     1.92    >4.5              Corrosion                             ______________________________________                                    

The analysis of the results leads to the following conclusions:

the electrodes of the invention are more stable than those of the prior art;

the electrodes with a TiN interlayer obtained both by plasma jet deposition and by ionic nitridization are more stable in all operating conditions;

the electrodes with a TiN interlayer obtained by gas (NH₃) nitridization are stable in those operating conditions where the fluoride content remains below 1000 ppm.

EXAMPLE 6

12 electrode samples comprising various interlayers based on intermetallic compounds comprising titanium nitrides (major component) and titanium carides (minor component) and having the same dimensions as those of Example 1 were prepared following the same pre-treatment procedure described in Example 1. Activation was subsequently carried out by

carbonitridization of the samples by thermal treatment in molten salts (in situ formation of the protective interlayer of titanium nitrides and carbides)

application of the electrocatalytic coating as described in Table. 6.1.

                  TABLE 6.1                                                        ______________________________________                                         Method of forming the interlayer and the electrocatalytic coating              Interlayer                                                                     Sample                                                                               Composition                                                                              Thickness          Electrocatalytic                            No.   % by weight                                                                              (micron) Method    coating                                     ______________________________________                                         6.1   TiN ≦ 80                                                                          0.8-1.5  Immersion in                                                                             Ta--Ir (64%), by                            a,b,c,d                                                                              TiC ≧ 20    molten salts:                                                                            from precursor                                                       NaCN +    salt paints as in                                                    Na.sub.2 CO.sub.3 +                                                                      Example 1                                                            Li.sub.2 CO.sub.3 (550° C.)                                             for 30 minutes                                        6.2   TiN ≧ 90                                                                          3-3.5    Immersion in                                                                             Ta--Ir (64%), by                            a,b,c,d                                                                              TiC ≦ 10    molten salts:                                                                            from precursor                                                       NaCN +    salt paints as in                                                    Na.sub.2 CO.sub.3 +                                                                      Example 1                                                            Li.sub.2 CO.sub.3 (550° C.)                                             for 90 minutes                                        6.3   TiN ≧ 90                                                                          5-5.3    Immersion in                                                                             Ta--Ir (64%), by                            a,b,c,d                                                                              TiC ≦ 10    molten salts:                                                                            from precursor                                                       NaCN +    salt paints as in                                                    Na.sub.2 CO.sub.3 +                                                                      Example 1                                                            Li.sub.2 CO.sub.3 (550° C.)                                             for 120 minutes                                       ______________________________________                                    

The samples thus prepared were subjected to electrochemical characterization as anodes in four types of electrolytes simulating the industrial operating conditions as shown in Table 6.2. For each type of operating conditions a comparison was made using reference samples prepared according to the prior art teachings as described in Example 1.

                  TABLE 6.2                                                        ______________________________________                                         Electrochemical characterization                                                                                  Operating                                   Series                                                                               Sample No.      Electrolyte  conditions                                  ______________________________________                                         M     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   6.1a→6.3a,                                                                              HF      50 ppm 40° C.                                   reference samples:                                                             A17, B17                                                                 N     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   6.1b→6.3b,                                                                              HF      300 ppm                                                                               50° C.                                   reference samples:                                                             A18, B18                                                                 O     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               1000 A/m.sup.2                                  6.1c→6.3c,                                                                              H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples:                                                             A19, B19                                                                 P     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               5000 A/m.sup.2                                  6.1d→6.3d,                                                                              H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples:                                                             A20, B20                                                                 ______________________________________                                    

The characterization comprised:

detecting the electrode potential as a function of the operating time

detecting the possible noble metal loss at the end of the test

visual inspection.

The results are summarized in Table 6.3

                  TABLE 6.3                                                        ______________________________________                                         Results of the characterization                                                         Potential V/NHE                                                       Electrolyte                                                                           Samples initial 100 h 1000 h                                                                               3000 h                                                                               Morphology                            ______________________________________                                         M      6.1a    1.74    1.80  1.83  1.89  No variation                                 6.2a    1.80    1.80  1.80  1.85  "                                            6.3a    1.81    1.80  1.81  1.88  No variation                                 A17     1.66    3.19              Corrosion                                    B17     1.67    2.41              Corrosion                             N      6.1b    1.80    1.81  1.84  1.88  No variation                                 6.2b    1.80    1.81  1.81  1.86  "                                            6.3b    1.81    1.82  1.82  1.82  "                                            A18     1.62    2.95              Corrosion                                    B18     1.66    2.26              Corrosion                             O      6.1c    1.83    1.89  1.90  1.95  No variation                                 6.2c    1.83    1.84  1.84  1.91  "                                            6.3c    1.84    1.85  1.84  1.92  "                                            A19     1.67    3.19              Corrosion                                    B19     1.74    2.61              Corrosion                             P      6.1d    1.91    1.94  1.97  2.38  No variation                                 6.2d    1.90    1.91  1.91  1.96  "                                            6.3d    1.92    1.94  1.93  1.94  "                                            A20     1.84    >6.0              Corrosion                                    B20     1.90    >5.0              Corrosion                             ______________________________________                                    

The analysis of the results leads to the following considerations

all the electrodes of the invention are more stable than those of the prior art;

in particular, the best performance was recorded by the samples prepared with the longest treatment time in the molten salt bath.

EXAMPLE 7

18 electrode samples having the dimensions of 40 mm×40 mm×2 mm, were prepared applying an interlayer based on tungsten, by plasma jet deposition of a tungsten powder having an average grain size of 0.5-1.5 micron. An electrocatalytic coating was then applied as described in Table 7.1.

                  TABLE 7.1                                                        ______________________________________                                         Method of application of the interlayer and electrocatalytic coating                   Interlayer                                                                     Thickness                                                              Sample No.                                                                             (micron) Electrocatalytic coating                                      ______________________________________                                         7.1a,b,c,d,e,f                                                                         15-25    Thermal decomposition of precursor salts of                                    Ta--Ir (64%) as in Example 1.                                 7.2a,b,c,d,e,f                                                                         30-40    Thermal decomposition of precursor salts of                                    Ta--Ir (64%) as in Example 1.                                 7.3a,b,c,d,e,f                                                                         70-80    Thermal decomposition of precursor salts of                                    Ta--Ir (64%) as in Example 1.                                 ______________________________________                                    

The samples thus prepared were subjected to electrochemical characterization as anodes in six types of electrolytes simulating the industrial operating conditions as shown in Table 7.2.

                  TABLE 7.2                                                        ______________________________________                                         Electrochemical characterization                                                                                  Operating                                   Series                                                                               Sample No.      Electrolyte  conditions                                  ______________________________________                                         M     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   7.1a→7.3a,                                                                              HF      50 ppm 40° C.                                   reference samples:                                                             A21, B21, 2.4 (Example 2).                                               N     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   7.1b→7.3b,                                                                              HF      300 ppm                                                                               50° C.                                   reference samples:                                                             A22, B22, 2.4 (Example 2).                                               O     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               1000 A/m.sup.2                                  7.1c→7.3c,                                                                              H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   reference samples:                                                             A23, B23, 2.4 (Example 2).                                               P     Present invention: from                                                                        H.sub.2 SO.sub.4                                                                       150 g/l                                                                               5000 A/m.sup.2                                  7.1d→7.3d,                                                                              H.sub.2 SiF.sub.6                                                                      1500 ppm                                                                              60° C.                                   reference samples:                                                             A24, B24, 2.4 (Example 2).                                               Q     Present invention: from                                                                        H.sub.2 SiF.sub.6                                                                      50 g/l 500 A/m.sup.2                                   7.1e→7.3e,              60° C.                                   reference samples:                                                             A25, B25, 2.4 (Example 2).                                               R     Present invention: from                                                                        HBF.sub.4                                                                              50 g/l 500 A/m.sup.2                                   7.1f→7.3f,              60° C.                                   reference samples:                                                             A26, B26, 2.4 (Example 2).                                               ______________________________________                                    

The characterization comprised:

detecting the electrode potential as a function of the operating time

detecting the possible noble metal loss at the end of the test

visual inspection.

The results are summarized in Table 7.3.

                  TABLE 7.3                                                        ______________________________________                                         Results of the electrochemical characterization                                         Potential V(NHE)                                                      Electrolyte                                                                           Samples initial 100 h 1000 h                                                                               3000 h                                                                               Morphology                            ______________________________________                                         M      7.1a    1.7     1.71  1.73  1.78  No variation                                 7.2a    1.71    1.70  1.70  1.71  "                                            7.3a    1.68    1.67  1.68  1.68  "                                            A21     1.63    3.05              Corrosion                                    B21     1.66    2.44              Corrosion                                    2.4     1.58    1.64  1.70  1.69  No variation                          N      7.1b    1.71    1.72  1.75  1.82  "                                            7.2b    1.70    1.70  1.69  1.69  "                                            7.3b    1.67    1.70  1.68  1.68  "                                            A23     1.63    2.89              Corrosion                                    B23     1.67    2.36              Corrosion                                    2.4     1.63    1.70  1.83  1.90  No variation                          O      7.1c    1.72    1.74  1.78  1.86  "                                            7.2c    1.70    1.70  1.72  1.72  "                                            7.3c    1.70    1.70  1.71  1.69  "                                            A24     1.66    3.47              Corrosion                                    B24     1.76    2.81              Corrosion                                    2.4     1.63    1.70  1.72  1.80  No variation                          P      7.1d    1.74    1.76  1.86  1.89  "                                            7.2d    1.73    1.75  1.75  1.75  "                                            7.3d    1.73    1.73  1.74  1.74  "                                            A24     1.84    3.05              Corrosion                                    B24     1.94    3.10              Corrosion                                    2.4     1.75    1.77  1.84  2.00  Initial                                                                        corrosion                             Q      7.1e    1.66    1.69  1.83  1.86  Initial                                                                        corrosion                                    7.2e    1.68    1.68  1.68  1.67  Initial                                                                        corrosion                                    7.3e    1.67    1.69  1.68  1.68  Initial                                                                        corrosion                                    A25     1.65    >4.0              Initial                                                                        corrosion                                    B25     1.68    >4.0              Corrosion                                    2.4     1.70    1.90  2.1         Corrosion                             R      7.1f    1.65    1.70  1.77  1.79  No variation                                 7.2f    1.67    1.67  1.68  1.69  "                                            7.3f    1.65    1.66  1.66  1.66  "                                            A26     1.66    >4.0              Corrosion                                    B26     1.70    >5.0              Corrosion                                    2.4     1.75    1.95  2.5         Corrosion                             ______________________________________                                    

The analysis of the results lead to the conclusions that all the samples according to the present invention are more stable than those prepared according to the prior art teachings, in particular, the electrodes provided with the tungsten interlayer are stable also in concentrated fluoboric or fluosilicic baths where the samples of the previous examples became corroded.

EXAMPLE 8

36 electrode samples having the dimensions of 40 mm×40 mm×2 mm, were prepared by applying an interlayer based on suicides, precisely tungsten silicide and titanium silicide, by plasma jet deposition after the same pretreatment as described in Example 1. An electrocatalytic coating was then applied as described in Table 8.1.

                  TABLE 8.1                                                        ______________________________________                                         Method of application of the interlayer and electrocatalytic coating           Interlayer                                                                             Compo-  Thickness         Electrocatalytic                             Sample No.                                                                             sition  (micron) Method   coating                                      ______________________________________                                         8.1a,b,c,d,e,f                                                                         WSi.sub.2                                                                              20-30    Plasma jet                                                                              Ta--Ir (64%), by                                                      deposition of                                                                           thermal                                                               WSi.sub.2 powder                                                                        decomposition                                                         (0.5-1.5 starting from                                                         micron)  precursor salt paints                                                          as in Example 1                              8.2a,b,c,d,e,f                                                                         WSi.sub.2                                                                              40-50    Plasma jet                                                                              Ta--Ir (64%), by                                                      deposition of                                                                           thermal                                                               WSi.sub.2 powder                                                                        decomposition                                                         (0.5-1.5 starting from                                                         micron)  precursor salt paints                                                          as in Example 1                              8.3a,b,c,d,e,f                                                                         WSi.sub.2                                                                              70-80    Plasma jet                                                                              Ta--Ir (64%), by                                                      deposition of                                                                           thermal                                                               WSi.sub.2 powder                                                                        decomposition                                                         (0.5-1.5 starting from                                                         micron)  precursor salt paints                                                          as in Example 1                              8.4a,b,c,d,e,f                                                                         TiSi.sub.2                                                                             20-30    Plasma jet                                                                              Ta--Ir (64%), by                                                      deposition of                                                                           thermal                                                               TiSi.sub.2 (0.5-1.5                                                                     decomposition                                                         micron)  starting from                                                         powder   precursor salt paints                                                          as in Example 1                              8.5a,b,c,d,e,f                                                                         TiSi.sub.2                                                                             40-50    Plasma jet                                                                              Ta--Ir (64%), by                                                      deposition of                                                                           thermal                                                               TiSi.sub.2 (0.5-1.5                                                                     decomposition                                                         micron)  starting from                                                         powder   precursor salt paints                                                          as in Example 1                              8.6a,b,c,d,e,f                                                                         TiSi.sub.2                                                                             70-80    Plasma jet                                                                              Ta--Ir (64%), by                                                      deposition of                                                                           thermal                                                               TiSi.sub.2 (0.5-1.5                                                                     decomposition                                                         micron)  starting from                                                         powder   precursor salt paints                                                          as in Example 1                              ______________________________________                                    

The samples thus prepared were subjected to electrochemical characterization as anodes in six types of electrolytes simulating industrial operating conditions as shown in Table 8.2. For each type of operating conditions a comparison was made with some reference samples prepared according to the prior art teachings as described in Example 1 and a sample of Example 2 of the invention (sample 2.4).

                  TABLE 8.2                                                        ______________________________________                                         Electrochemical characterization                                                                                  Operating                                   Series                                                                               Sample No.      Electrolyte  Conditions                                  ______________________________________                                         M     8.1a→8.3a,                                                                              H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   reference samples:                                                                             HF      50 ppm 40° C.                                   A27, B27, 2.4 (Example 2)                                                N     8.1b→8.3b,                                                                              H.sub.2 SO.sub.4                                                                       150 g/l                                                                               500 A/m.sup.2                                   reference samples:                                                                             HF      300 ppm                                                                               50° C.                                   A28, B28, 2.4 (Example 2)                                                O     8.1c→8.3c,                                                                              H.sub.2 SO.sub.4                                                                       150 g/l                                                                               1000 A/m.sup.2                                  reference samples:                                                                             H.sub.2 SiF.sub.6                                                                      1000 ppm                                                                              60° C.                                   A29, B29, 2.4 (Example 2)                                                P     8.1d→8.3d,                                                                              H.sub.2 SO.sub.4                                                                       150 g/l                                                                               5000 A/m.sup.2                                  reference samples:                                                                             H.sub.2 SiF.sub.6                                                                      1500 ppm                                                                              60° C.                                   A30, B30, 2.4 (Example 2)                                                Q     Present invention: from                                                                        H.sub.2 SiF.sub.6                                                                      50 g/l 500 A/m.sup.2                                   8.1e→8.3e,              60° C.                                   reference samples:                                                             A31, B31, 2.4 (Example 2)                                                R     8.1f→8.3f,                                                                              HBF.sub.4                                                                              50 g/l 500 A/m.sup.2                                   reference samples:             60° C.                                   A32, B32, 2.4 (Example 2)                                                ______________________________________                                    

The characterization comprised:

detecting the electrode potential as a function of the operating time

detecting the possible noble metal loss at the end of the test

visual inspection.

The results are summarized in Table 8.3.

                  TABLE 8.3                                                        ______________________________________                                         Results of the electrochemical characterization                                         Potential V(NHE)                                                      Electrolyte                                                                           Samples initial 100 h 1000 h                                                                               3000 h                                                                               Morphology                            ______________________________________                                         M      8.1a    1.74    1.74  1.78  1.81  No variation                                 8.2a    1.72    1.73  1.75  1.75  No variation                                 8.3a    1.70    1.71  1.71  1.72  No variation                                 8.4a    1.75    1.75  1.80  1.84  No variation                                 8.5a    1.74    1.74  1.77  1.77  No variation                                 8.6a    1.69    1.71  1.70  1.73  No variation                                 A27     1.63    3.05              Corrosion                                    B27     1.69    2.44              Corrosion                                    2.4     1.58    1.64  1.70  1.69  No variation                          N      8.1b    1.72    1.76  1.77  1.82  No variation                                 8.2b    1.71    1.71  1.71  1.74  No variation                                 8.3b    1.70    1.71  1.72  1.72  No variation                                 8.4b    1.77    1.78  1.77  1.90  No variation                                 8.5b    1.72    1.73  1.73  1.73  No variation                                 8.6b    1.73    1.72  1.70  1.72  No variation                                 A28     1.62    2.89              Corrosion                                    B28     1.71    2.36              Corrosion                                    2.4     1.63    1.70  1.83  1.90  No variation                          O      8.1c    1.75    1.75  1.79  1.84  No variation                                 8.2c    1.70    1.70  1.75  1.75  No variation                                 8.3c    1.70    1.73  1.73  1.74  No variation                                 8.4c    1.76    1.81  1.82  1.86  No variation                                 8.5c    1.72    1.76  1.77  1.79  No variation                                 8.6c    1.72    1.75  1.76  1.77  No variation                                 A29     1.67    3.47              Corrosion                                    B29     1.76    2.81              Corrosion                                    2.4     1.63    1.70  1.72  1.80  No variation                          P      8.1d    1.75    1.76  1.79  1.90  No variation                                 8.2d    1.74    1.74  1.76  1.77  No variation                                 8.3d    1.75    1.75  1.75  1.78  No variation                                 8.4d    1.76    1.77  1.78  1.88  No variation                                 8.5d    1.74    1.76  1.75  1.77  No variation                                 8.6d    1.76    1.77  1.77  1.79  No variation                                 A30     1.84    3.05              Corrosion                                    B30     1.94    3.10              Corrosion                                    2.4     1.75    1.77  1.84  2.00  Initial                                                                        corrosion                             Q      8.1e    1.68    1.68  1.75  1.84  No variation                                 8.2e    1.67    1.67  1.71  1.74  No variation                                 8.3e    1.65    1.70  1.70  1.70  No variation                                 8.4e    1.66    1.66  1.74  1.89  No variation                                 8.5e    1.71    1.70  1.73  1.76  No variation                                 8.6e    1.73    1.72  1.73  1.78  No variation                                 A31     1.64    >2.0              No variation                                 B31     1.68    >4.0              Corrosion                                    2.4     1.70    1.90  2.1         Corrosion                                    (Ex. 2)                                                                 R      8.1f    1.66    1.67  1.68  1.92  No variation                                 8.2f    1.67    1.67  1.71  1.73  No variation                                 8.3f    1.70    1.72  1.72  1.73  No variation                                 8.4f    1.70    1.72  1.78  1.89  No variation                                 8.5f    1.74    1.74  1.73  1.73  No variation                                 8.6f    1.70    1.70  1.72  1.75  No variation                                 A32     1.66    >4.0              Corrosion                                    B32     1.70    >5.0              Corrosion                                    2.4     1.75    1.95  2.5         Corrosion                                    (Ex. 2)                                                                 ______________________________________                                    

The analysis of the results lead to the following conclusions:

all the samples according to the present invention are more stable than those prepared according to the prior art teachings;

in particular, the electrodes provided with the titanium or tungsten silicide interlayer are stable also in concentrated fluoboric or fluosilicic baths wherein the samples of the previous example 2 became corroded.

The above discussion clearly illustrates the distinctive features of the present invention and some preferred embodiments of the same. However, further modifications are possible without departing from the scope of the invention, which is limited only by the following appended claims. 

We claim:
 1. An anode for electrometallurgical process using acid solution containing fluorides, consisting essentially of a titanium substrate provided with a protective interlayer and an outer electrocatalytic coating for oxygen evolution wherein the said interlayer is made of tungsten.
 2. In the method for electroplating a metal onto a cathode the improvement comprises using as the anode the anode of claim
 1. 3. The method of claim 2 wherein the metal being plated is selected from the group consisting of chromium, zinc, gold, and platinum.
 4. An anode for electrometallurgical processes using acid solutions containing fluorides or fluoride-complex anions, consisting essentially of a titanium substrate provided with a protective interlayer and an outer electrocatalytic coating for oxygen evolution wherein the said interlayer is selected from the group consisting of oxides oxyfluorides and mixed oxides of at least one metal selected from the group consisting of chromium, yttrium, cerium, lanthanides, titanium and niobium.
 5. The anode of claim 4 wherein the interlayer further contains minor amount of platinum group metals, or as a mixture thereof.
 6. The anode of claim 5 wherein said metals of the platinum group are platinum, palladium and iridium.
 7. Anode for electrochemical processes using acid solutions containing fluorides or fluoride-complex anions, comprising a titanium substrate provided with a protective interlayer and an electrocatalytic coating for oxygen evolution characterized in that said interlayer is made of a metalloceramic mixture.
 8. The anode of claim 7 wherein said metalloceramic mixture contains chromium as the metal component and chromium oxide as the ceramic component.
 9. An anode for electrometallurgical processes using acid solutions containing fluorides or fluoride-complex anions, consisting essentially of a titanium substrate provided with a protective interlayer and an outer electrocatalytic coating for oxygen evolution wherein the said interlayer is made of intermetallic compounds or as a mixture thereof.
 10. The anode of claim 9 wherein the said intermetallic compounds are selected from the group consisting of nitrides, carbides and silicides.
 11. The anode of claim 10 wherein the said intermetallic compounds are selected from the group consisting of titanium nitrides, carbides and silicides and tungsten silicides.
 12. In the method for electroplating a metal onto a cathode the improvement comprises using as the anode the anode of claim
 7. 